Motor-driven power steering device

ABSTRACT

An electric power steering apparatus can correspond to high output ability, and has a power transmission mechanism of an electric driving section capable of achieving predetermined slip torque in spite of change of operating temperature. Also, a resin gear for power transmission is formed by joining a core metal and a resin with strong bonding force, and has preferable heat-dissipation property. This joining is performed by way of chemical bond according to composite molding technique or adhesive.

TECHNICAL FIELD

The present invention relates to an electrically power assistingsteering apparatus.

BACKGROUND ARTS

In the operation of an electrically power assisting steering apparatus(hereinafter called as an EPS apparatus) wherein turning force of anelectric motor becoming auxiliary steering torque is decelerated by agear device to transmit it to an output shaft, thereby to assist adriver's power for rotating a steering wheel to steer wheels, e.g., whenthe steering wheel is rotated up to a stroke end of a rack and thesteering is abruptly stopped, the motor can not stop immediately due toinertia force and occasionally, excessive torque is produced in thedecelerating gear portion. In order to protect decelerating gearsagainst the excessive torque, conventionally, a torque limiter using afriction plate is provided between a motor and a worm, or a torquelimiter using a ring member for applying elastic force is providedbetween an output shaft and a worm wheel. Thereby, at the time ofgeneration of such excessive torque, slip is developed between both ofthem to prevent transmission of the excessive torque.

In an EPS apparatus, generally, a worm is formed of iron material, and aworm wheel is made by forming a gear portion of resin materialintegrally on the outer peripheral surface of a core portion of solidiron material by way of adhesion, fusion or injection molding. Also, anoutput shaft having the worm wheel mounted thereon is formed of ironmaterial the same as the core metal.

In this field of the art, the ring member for applying elastic forceused for the torque limiter provided between the output shaft and theworm wheel is called as a rotation transmitting member or a torquesetting member, etc., or is known as a tolerance ring (trade name:Rencol Tolerance Rings).

As mentioned above, in the conventional EPS apparatus, since the coremetal portion of the worm wheel constituting the gear device is formedof solid iron material, there are the following problems. First, whenattempting to correspond to achieving high output of the EPS apparatus,it is necessary to make the diameter of the worm wheel large for highstrength and high module ability. However, in that case, the weight ofthe core metal portion is increased and inertial force in steering isincreased, so that a driver's feeling in rotating the steering wheel andcutting of the steering wheel are deteriorated. Also, in compliance withthe enlargement of the diameter of the core metal portion, when thediameter of the resin gear portion is enlarged, accuracy of the gearsmight be worsened to cause increase of noise of the gears in operationsince dimensional change of resin due to moisture absorption andtemperature is large. Also, the strength of the gear might be loweredsince there is the danger of occurring voids in the resin portion due toinjection molding. Thus, in the conventional EPS apparatus, there areproblems in making the diameter of the worm wheel large.

Next, by comparison on the basis of ordinary temperature, the resin gearportion has characteristics in that its strength is lowered under hightemperature and is heightened under low temperature. However, theconventional output shaft and the core metal portion are made of ironmaterial and their linear expansion coefficients are approximately thesame, so that the limit torque of the torque limiter is constant totemperature. Therefore, the limit torque of the torque limiter needs tobe set at the maximum operating temperature, and needs to be set low,whereby the difference between the limit torque and the transmissiontorque was small. When the ring is worn and the limit torque is loweredbecause of long-term use of the EPS apparatus, the torque transmissionmight not be carried out, and thereby the designing is difficult.

For example, a worm wheel is disclosed in the Japanese Utility ModelPublication No. 2556890 and the Japanese Patent Application Laid-OpenNo. 7-215227. As shown in FIGS. 15A and 15B, the worm wheel has a coremetal X19 and a resin ring X20. The entire outer peripheral surface ofthe core metal 19X is formed in the shape of a gear with irregularitiesX19 a constituted of rotational direction stopping serrations X19 b andaxial direction stopping projections X19 c. The resin ring X20 withteeth on its outer peripheral surface is fitted on the irregularitiesX19 a of the core metal X19. As the thickness of the resin ring X20 isproperly set, it is possible to maintain proper backlash between thegears even though atmospheric temperature is changed.

Also, in a worm wheel disclosed in the Japanese Patent ApplicationLaid-Open No. 11-192955, an approximately gear-shaped core metal is usedfor enhancement of the strength of the teeth, and the gear-shaped rimportion of the core metal is covered by a resin by way of injection suchthat the cross section of the resin in the axial direction becomes arectangle. Then, the binding of the rim portion and the resin depends onthe strength of the cooled and hardened thin resin layer.

However, in the conventional worm wheel of the Japanese Utility ModelPublication No. 2556890 and the Japanese Patent Application Laid-OpenNo. 7-215227, the resin ring X20 is fitted on the irregularities X19 aformed on the outer peripheral surface of the metallic core metal 19X,but since the mechanical strength of the resin is inferior to that ofmetal, the module needs to be enlarged in comparison with a metal gear,and the thickness of the teeth needs to be made larger than that of themetallic portion, whereby there occurs a problem that the deceleratingmechanism is enlarged.

Also, the resin with the large thickness is inferior in heat-dissipationability, and so the thickness of the resin is set appropriately, butthere is a problem in that wear of the resin is increased due to heatproduced in the engagement of the gears. Especially, as shown in FIG.16, when the worm wheel is incorporated in the EPS apparatus anddisposed in an engine room, the temperature condition is severe and itis difficult to secure the durability under high temperatureenvironment. (Description of the structure of an EPS apparatus in FIG.16 will be made later in the description of the preferred embodimentsand then omitted here).

Further, since the binding power of the core metal X19 and the resinring X20 depends on mechanical catching of the irregularities 19 a, itis necessary to form the thickness of the resin ring X20 large so as tomake the resin ring X20 withstand the slipping-off force. In case ofusing nylon family resin, as it has water absorptive property, thelarger the volume of the resin becomes, the larger its dimensionalchange caused by expansion due to water absorption becomes. However, inthe EPS apparatus, as a driver rotates a steering wheel, a steeringassisting force is transmitted via the decelerating mechanism, wherebythe rotational directions in the decelerating mechanism are reversedintricately. Therefore, in order to prevent noise from the teethsurfaces of the gears, it is necessary to control such that the backlashbecomes extremely small. As mentioned above, when the volume of theresin is large, its dimensional change due to water absorption isconsiderably large. Therefore, there is a problem in that when the resinabsorbs water and expands to the extent that the backlash is lost, theoperability is deteriorated.

Further, in the worm wheel disclosed in the Japanese Patent ApplicationLaid-Open No. 11-192955, external force generated due to the engagementof the gears acts on the thin resin layer as tensile stress, so that theresin is easily broken and it is difficult to obtain strength. Also, ina case where the core metal is used as the insert in an injectionmolding, when the high temperature resin is cooled and contracted,residual tensile stress is produced. In this case, the joining of theresin and the core metal relies on mechanical catching, and if only aone portion of the resin is broken, the broken-out surface is enlargeddue to the internal stress, so that the binding force is lost.

Two types of resin worm wheels are mass-produced: resin of MC nylon typeand resin containing glass fiber type. In the case of the former MCnylon worm wheel, while a tube-like MC nylon and a core metal withirregularities (knurls) as described above on its outer peripheralsurface are subjected to high frequency induction heating, their joiningportions are tight-contacted and bonded with adhesive. However, the costof the MC nylon is very high, and its heat resistance is up to 80° C.

On the other hand, the latter worm wheel is produced byinjection-molding the resin on irregularities of the outer peripheralsurface of a core metal which is fairly lager than that of the MC nylontype, and thereby fixing the resin by the irregularities mechanically.In this case, since heated resin is injected, the resin is contractedwhen cooled, and tensile stress acts on the interface between the resinand the core metal. Accordingly, if even a one portion of voids and aweld line is broken, a crack is spread and the fixing force between thecore metal and the resin is lost. Further, although the heat resistanceof the resin is up to 120° C., there is a problem in that operability isreduced due to expansion of the resin caused by water absorption.

DISCLOSURE OF THE INVENTION

It is a first object of the present invention to provide an electricpower steering apparatus equipped with a power transmission mechanism ofan electric driving section capable of corresponding to high output, andachieving predetermined slip torque in accordance with change inoperating temperature.

In order to achieve the first object of the present invention, in an EPSapparatus provided with a torque limiter having a ring member forapplying elastic force between a worm wheel and an output shaft, theoutput shaft is made of iron material, a gear portion of the worm wheelis made of resin material, and a core metal portion thereof is made ofmetallic material whose specific gravity is small and linear expansioncoefficient is large with respect to the iron material thereby to setlimit torque of the torque limiter low under high temperature and highunder low temperature.

It is a second object of the present invention to provide an EPSapparatus having a worm wheel incorporated in a transmission pathway ofauxiliary steering force in which a core metal and a resin are bondedwith strong bonding power, heat dissipation ability is preferable, andminiaturization is possible by the use of the minimum amount of resin.

In order to achieve the second object, in an EPS apparatus having a wormwheel incorporated therein for transmitting driving force of a motor asauxiliary steering force together with a worm gear, the presentinvention proposes the worm wheel formed by joining a thin resin to theentire outer peripheral surface of teeth portion of a gear-shaped coremetal by way of chemical bond according to composite molding technique,or by means of adhesive. Further, the core metal can be made of aluminumalloy or copper alloy. As structured above, the bonding of the teethportion of the core metal and the resin becomes strong and even thoughthe thickness of the resin is made small, it is hard to receive tensilestress due to the engagement of the gears. Therefore, the resin isprevented from breaking and the amount of resin can be reduced, wherebydimensional change of the gear due to water absorption can be reducedconsiderably. Since the core metal is made of aluminum alloy or copperalloy, heat is easily diffused and it can be used under comparativelyhigh temperature. Especially, when the core metal is made of aluminumalloy and incorporated in the EPS apparatus as the worm wheel, it hasthe same thermal expansion coefficient as the aluminum die-cast gearhousing, change of backlash of the gears due to thermal expansion isreduced extremely, making it possible to maintain preferable operationof the gears constantly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial sectional view showing an essential portion of anelectric power steering apparatus of a first embodiment of the presentinvention, i.e., a power transmission mechanism in an electric drivingsection;

FIG. 2 is a sectional view of the power transmission mechanism in FIG. 1taken along the X—X line;

FIG. 3 is a perspective view of a ring member for applying elasticforce;

FIG. 4 is a partially enlarged sectional view showing the mounted stateof the ring member for applying elastic force;

FIG. 5 is a diagram showing the relationship between the deformation ofthe ring member in the radial direction and the force thereof in theradial direction;

FIG. 6 is a diagram showing the relationship between torque andtemperature change;

FIG. 7A is a side view of a worm wheel of a second embodiment of thepresent invention;

FIG. 7B is a sectional view of the worm wheel in FIG. 7A taken alonglines indicated by arrows C and D;

FIG. 8 is a perspective view of the worm wheel in FIG. 7A;

FIG. 9 is a sectional view of an EPS apparatus incorporating the wormwheel of FIG. 7A;

FIG. 10 is a sectional view of an EPS apparatus incorporating the wormwheel of FIG. 7A with a torque limiter showing a third embodiment;

FIG. 11A is a side view of a worm wheel showing a fourth embodiment;

FIG. 11B is a sectional view of the worm wheel in FIG. 11A taken alonglines indicated by arrows C and D;

FIG. 12 is a perspective view of the worm wheel in FIG. 11A;

FIG. 13A is a side view of a worm wheel showing a fifth embodiment;

FIG. 13B is a sectional view of the worm wheel in FIG. 13A taken alongthe lines indicated by the arrows C and D;

FIG. 14A is a side view of a worm wheel showing a sixth embodiment;

FIG. 14B is a sectional view of the worm wheel in FIG. 14A taken alonglines indicated by arrows C and D;

FIG. 15A is a side view of a conventional worm wheel;

FIG. 15B is a sectional view of the worm wheel of FIG. 15A in the radialdirection; and

FIG. 16 is a sectional view of an EPS apparatus incorporating the wormwheel in FIG. 15A.

EMBODIMENTS OF THE INVENTION

The structure of a first embodiment of the present invention will bedescribed by reference to drawings. FIG. 1 is a partial sectional viewof the essential part of an EPS apparatus of the first embodiment, thatis, the power transmission mechanism of an electric driving section inthe axial direction. FIG. 2 is a sectional view of the powertransmission mechanism taken along the X—X line of FIG. 1. In FIGS. 1and 2, a reference number 1 is an electric motor for assisting insteering. 2 is an output shaft of the electric motor 1. 3 is a wormconnected to the output shaft 2 by means of serration engagement or ajoint, and 3 a is the gear portion of the worm 3. 4 is a worm wheelhaving a core metal portion 4 b with a hole 4 c in its center and a gearportion 4 a formed integrally on the outer peripheral surface of thecore metal portion 4 b. The gear portion 3 a of the worm 3 is meshedwith the gear portion 4 a of the worm wheel 4. 5 is an input shaftconnected to a steering wheel (not shown). 6 is a cylindrical housingfor supporting the input shaft 5 rotatably around its axis. 7 is anoutput shaft. 8 is bearings for rotatably supporting the output shaft 7.9 is a housing for supporting the bearings 8. 10 is a ring membermounted between the outer peripheral surface of the output shaft 7 andthe hole 4 c of the core metal portion 4 b of the worm wheel 4 so as toapply elastic force in the radial direction. 11 and 12 are bearings forsupporting the worm 3, and 13 is a housing. The input shaft 5 and theoutput shaft 7 are connected via a torsion bar(not shown).

Thus, the electric driving section of the present EPS apparatus isconstituted of the above power transmission mechanism, and when theoutput shaft 2 is rotated by the electric motor 1, the worm wheel 4 isrotated via the worm 3, and the power of the electric motor 1 istransmitted to the output shaft 7.

In the above power transmission mechanism, the output shaft 7 is formedof iron material. On the other hand, the core metal portion 4 b of theworm wheel 4 is formed of material such as aluminum, and the like, atleast, with specific gravity smaller than that of the iron material ofthe output shaft 7, and a linear expansion coefficient larger than thatof the iron material. Also, the gear portion 4 a is formed of syntheticresin material such as nylon, and the like, having hardness andexcellent formability. Thus, in the worm wheel 4 of the powertransmission mechanism of the present EPS apparatus, lightening ofweight, enhancement of strength and prevention of noise are achieved.

Further, the aluminum core metal portion 4 b of the worm wheel 4 and theresin gear portion 4 a are integrally formed by means of adhesive, orthe outer peripheral surface of the core metal portion 4 b is subjectedto emboss, knurl or serration processing so as to have irregularities.Thus, precaution is taken so as to have the outer peripheral surface ofthe core metal portion 4 b adequately caught to the inner peripheralsurface of the resin gear portion 4 a. Therefore, when the gear portion4 a is mounted integrally on the outer peripheral surface of the coremetal portion 4 b to form the worm wheel 4 by means of injectionmolding, fusion, press fit or the like, the strength of both is obtainedsufficiently, thereby slip torque and slipping-off strength are secured.

The ring member 10 is formed of, for example, a known member as shown inthe perspective view of FIG. 3. That is, the ring member 10 is formed bybending a thin metallic plate with a space C approximatelycylindrically. The inner peripheral surface of the ring member 10 isformed with a plurality of grooves 10 a in the axial direction, whilethe outer peripheral surface thereof is formed with a plurality ofprojections 10 b having height in the radial direction. The ring member10 is mounted on the output shaft 7 such that the grooves 10 a areengaged with projections (not shown) provided on the outer peripheralsurface of the output shaft 7. The projections 10 b are formed of springsteel or the like having elasticity. When the worm wheel 4 is mounted onthe projections 10 b, as shown in an enlarged sectional view in FIG. 4,the projections 10 b are brought into contact with the inner peripheralsurface of the core metal portion 4 b, applying appropriate frictionalforce between the ring member 10 and the output shaft 7. Thus, a torquelimiter is constituted of the ring member 10 for applying elastic forcein the radial direction.

In the EPS apparatus, the limit torque of this torque limiter isdetermined by taking into consideration the difference between thelinear expansion coefficients of the output shaft 7 and the worm wheel4, especially its core metal portion 4 b. While the output shaft 7 isformed of iron material, the core metal portion 4 b is formed ofaluminum material whose linear expansion coefficient is over twice thatof the iron material. Therefore, due to the rise and fall of operatingtemperature, the distance between them, that is, the change in thelinear dimension in the radial direction is varied. As shown in FIG. 4,if the distance between the outer peripheral surface of the output shaft7 and the inner peripheral surface of the core metal portion 4 b is D,the distance D becomes the maximum distance D (max) when the operatingtemperature is the maximum temperature, and it becomes the minimumdistance D (min) when it is the minimum temperature. Accordingly, thelimit torque is set such that at the maximum operating temperature,namely, in the case of the maximum distance D (max), the limit torquecoincides with the predetermined interference of the ring member 10.

The method of setting the limit torque will be described hereafter byreference to FIG. 5 showing a relationship between the amount ofdeformation of the ring member 10 in the radial direction (horizontalaxis) and the force (torque) in the radial direction (vertical axis). InFIGS. 5, A and B respectively represent the width of change of the limittorque of the torque limiter and the width of change of interference dueto change in temperature. Also, points T0, T1 and T2 respectivelyrepresent the set value at the ordinary temperature, the set value atthe maximum operating temperature and the set value at the minimumoperating temperature. Further, lines Na and Nb respectively representthe rotational energizing force of the electric motor 1 and the breakingtorque of the resin material. As shown in the drawing, the width A ofchange in the limit torque of the torque limiter is set in the range ofthe width B of change of the interference due to the change intemperature set within a plastic deformation area between the ordinaryused specific rotational energizing force Na and the breaking torque Nbof the resin material changeable due to temperature, i.e., between theset value T1 at the maximum operating temperature and the set value T2at the minimum operating temperature.

FIG. 6 is a detailed schematic drawing, showing the relationship betweenthe temperature change (horizontal axis) and the torque (vertical axis).In the drawing, lines No and Np respectively represent the limit torqueof the torque limiter of the present EPS apparatus and the limit torqueof the torque limiter of a conventional EPS apparatus constituted of aniron output shaft and an iron core metal portion. As shown in thedrawing, while the limit torque Np of the torque limiter of theconventional EPS apparatus is constant to temperature as describedabove, the limit torque No of the torque limiter of the present EPSapparatus is set between the rotational energizing force Na of theelectric motor 1 changeable due to temperature and the breaking torqueNb of the resin material.

Thus, the present EPS apparatus is formed so as to obtain the sliptorque according to the operating temperature.

Also, the difference of the operating temperature has an influence onthe resin gear portion 4 a, and the strength thereof is increased underlow temperature, while it is lowered most at the maximum operatingtemperature.

The above first embodiment describes the power transmission mechanismconstructed by a combination of the worm and the worm wheel, but thepower transmission mechanism may be constructed by a combination ofvarious gears such as a hypoid gear, a bevel gear, helical gear, etc.

In the electric power steering apparatus of the first embodiment, sincethe core metal portion of the worm wheel constituting the powertransmission mechanism of the electric driving section is formed of themetallic material with the specific gravity smaller than that of theiron material, the weight of the worm wheel is reduced. Thereby, eventhough the diameter of the worm wheel is enlarged, inertia force at thetime of steering is reduced and a driver's feeling of steering at thetime of a cut of the steering wheel is enhanced, making it possible tocorrespond to high output of the EPS apparatus. When the diameter of thethus lightened core metal portion is enlarged, the diameter of the resingear portion can be reduced, thereby canceling problems concerningformability such as dimensional change, voids, etc. due to theenlargement of the diameter of the resin portion. When aluminum whosespecific gravity is about one-third of that of the iron material ischosen for the material of the core metal portion, these effects can beimproved furthermore.

The structure of another embodiment of the present invention will bedescribed by reference to drawings.

FIG. 7A is a side view of a worm wheel of a second embodiment of thepresent invention. FIG. 7B is a sectional view of the worm wheel takenalong a line indicated by allows C and D in FIG. 7A. FIG. 8 is aperspective view of the worm wheel in FIG. 7A. FIG. 9 is a sectionalview of an EPS apparatus incorporating the worm wheel of FIG. 7A. FIG.10 is a sectional view of an EPS apparatus incorporating the worm wheelof FIG. 7A with a torque limiter, showing a third embodiment. FIG. 11Ais a side view of a worm wheel showing a forth embodiment. FIG. 11B is asectional view of the worm wheel in FIG. 11A taken along a lineindicated by arrows C and D. FIG. 12 is a perspective view of the wormwheel in FIG. 11. FIG. 13A is a side view of a worm wheel showing afifth embodiment. FIG. 13B is a sectional view of the worm wheel in FIG.13A taken along a line indicated by arrows C and D. FIG. 14A is a sideview of a worm wheel showing a sixth embodiment. FIG. 14B is a sectionalview of the worm wheel taken along a line indicated by arrows C and D inFIG. 14A.

In FIGS. 7A, 7B and 8, a worm wheel 101 has a core metal 102 and asynthetic resin 103. The outer peripheral surface of the core metal 102is formed in the shape of a helical gear, and the resin 103 is joined tothe entire teeth surface of the core metal 102.

The joining of the resin 103 to the teeth surface of the core metal 102is performed by way of the chemically bonding composite moldingtechnique, e.g., the TRI composite making of Toa Denka Corporation. Thecompletion of the joining results in formation of a helical gear whoseentire tooth surface is formed of the resin 103. Thereafter, machiningis carried out by the use of a hob cutter to finish the worm wheel 101.

The above TRI composite making is the technique to bind metal such asaluminum alloy, steel alloy, etc., and plastic in a metal mold by way ofchemical bond, applying electrochemical special metal surface processingand insert injection molding. The power of bonding is solid and adhesiveis not used, so that minute components can be freely designed.

In the joining process of the core metal 102 and the resin 103 by way ofthe TRI composite making, first, the teeth surface (the entire outerperipheral surface) of the core metal 102 is subjected to organicplating processing to cause chemical reaction in an inner portiondirectly under the teeth surface. Next, the core metal 102 is fitted inan injection metal mold, and molten resin is injected in the metal moldto perform insert molding. Thereby, the resin 103 is joined to the teethsurface of the core metal 102.

FIG. 9 shows a pinion assist-type EPS apparatus incorporating the wormwheel 101 formed by the use of the TRI composite making. In the drawing,a steering wheel (not shown) is connected to the left end portion of aninput shaft 107, and the other end portion of the input shaft 107 isconnected to an output shaft (pinion shaft) 106 via a torsion bar 111.The core metal 102 of the worm wheel 101 is fitted and fixed on aconnecting portion of the output shaft 106, and aluminum alloy or copperalloy is used for the core metal 102. The worm wheel 101 is meshed witha worm gear 105,and these are accommodated in a gear housing 115. Apinion 106 a is formed on the other side of the output shaft 106 andengaged with a rack 109. When the output shaft 106 is rotated, the rack109 is shifted in a rectangular direction to the paper of the drawing.In the engaging portion of the rack 109 and the pinion 106 a, the rack109 is pressed and supported by a rack guide 112 biased elastically bymeans of a spring 108. The rack 109 is linked to wheels via a tie rodand a link mechanism (not shown). When the steering wheel is operatedand the input shaft 107 is rotated leftward or rightward, the rack 109is shifted leftward or rightward to turn the wheels leftward orrightward. The input shaft 107 and the output shaft 106 are respectivelypositioned and rotatably supported by a ball bearing 113, and ballbearings 110 and 114.

The worm wheel 101 is formed such that the resin 103 is joined only tothe teeth surface of the core metal 102 by way of chemical bond.Therefore, even though the layer of the resin 103 is thin, the force ofthe engagement of the gears bears as shearing stress of the joiningsurface, there is little fear of breaking. Also, in the injectionmolding, when high temperature resin is cooled and contracted, residualtensile stress is generated. However, breaking of a one portion causesno enlargement of the broken-out surface, so that the bonding power willnot be lost.

As the aluminum alloy or the copper alloy with large thermalconductivity is used for the core metal 102, heat produced due to theengagement of the gears is easily diffused. Thereby, it is possible touse it in comparatively high temperature environment such as an engineroom. Especially, when the aluminum alloy is used, the coefficient ofthermal expansion thereof is the same as that of the aluminum die-castgear housing 115 (refer to FIG. 9), change in gear backlash due to thethermal expansion is extremely reduced, making it possible to obtain thepreferable operation of the gears. Also, as compared to the case wheregear teeth are formed on a ring-like resin, the amount of used resin canbe limited to a minimum. Therefore, it is possible to reduce thedimensional change due to water absorption, and smooth operation of thegears can be maintained. Since the core metal 102 takes up the most partof the whole volume of the worm wheel 101, the strength of the gear canbe obtained by means of the core metal 102, whereby a gear module can bereduced in size. Therefore, it is possible to realize miniaturization ofthe apparatus and low cost.

In the second embodiment, the joining of the core metal 102 and theresin 103 is performed by way of the chemical bond according to the TRIcomposite technology, but is not limited thereto, and even if those arejoined by the use of appropriate adhesive with high adhesive strengthand high heat resistance, the same effect can be expected. However, inthis case, the bonding power is inferior to that of the chemical bond.

Also, as the aluminum alloy or copper alloy is utilized for the materialof the core metal 102, the coefficient of thermal expansion thereof isdifferent from that of the iron output shaft 106, so that it is notpossible to obtain sufficient holding power by way of the conventionalpress fit. Therefore, as shown in FIG. 9, the output shaft 106 isinserted in an inner-fitting portion (hole) 116 of the core metal 102 byway of serration press fit.

FIG. 10 shows structure of a third embodiment. In this embodiment, atorque limiter is provided between the worm wheel 101 and the outputshaft 106 of an EPS apparatus, and other than this portion is the sameas that of the EPS apparatus in the second embodiment. As the torquelimiter mounted between the hole of the core metal 102 of the worm wheel101 and the outer peripheral surface of the output shaft 106, a knowntolerance ring 117 for applying elastic force in the radial direction isused, whereby, at the time of occurrence of excessive torque, slip isdeveloped between the worm wheel 101 and the output shaft 106, andtransmission of the excessive torque can be prevented. With respect tothe output shaft 106 formed of the iron material, as mentioned above,when the core metal 102 is formed of material with small specificgravity and a large coefficient of thermal expansion, e.g., preferablyaluminum alloy (or copper alloy), the limit torque of the torque limiteris set low under high temperature and it is set high under lowtemperature, whereby the predetermined slip torque can be obtained inaccordance with change in operating temperature. As above, it ispossible to take the structure combining the worm wheel of the presentinvention and the torque limiter. Accordingly, in addition to theeffects described in the second embodiment, the EPS apparatus of thisembodiment can prevent excessive torque from transmitting.

Next, a fourth embodiment will be described by reference to FIGS. 11A,11B and FIG. 12. A worm wheel 130 of the fourth embodiment isapproximately the same as that of the second embodiment, and identicalmembers are designated with identical numbers. The different point isthat a resin 104 is attached not to the entire teeth surface of the coremetal 102 but to a portion thereof.

At the time of the resin injection of the worm wheel 130, a portion 102a of the teeth surface is fitted in an injection metal mold so as toadjust phases, and the resin is joined to a remaining portion 102 b ofthe teeth surface. Thereafter, the worm wheel 130 is subjected to a wormteeth cutting processing by means of the hob cutter. The other formationprocessing is the same as that of the second embodiment, and then thedescription thereof will be omitted. In this case, the volume of theresin 104 is smaller than that of the resin 103, so that the dimensionalchange due to water absorption can be reduced.

FIGS. 13A and 13B show a fifth embodiment. A worm wheel 140 of thisembodiment is approximately the same as that of the second embodiment,and same numbers are given to same members. A different point is that aone side of the core metal 102 in the axial direction is covered withthe resin 103 having a constant thickness by way of the TRI compositemaking technolory. Accordingly, when burrs produced in the teeth cuttingprocessing by means of the hob cutter are removed by a lathe, itsdeburring can be performed easily.

A sixth embodiment is shown in FIGS. 14A and 14B. A worm wheel 150 ofthis embodiment is approximately the same as that of the secondembodiment, and identical members are designated with identical numbers.The different point is that a portion other than a bearing portion 118 aof the core metal 118 is formed thinly. Thereby, its heat dissipationability is further improved, and lightening is achieved.

The above second, fourth, fifth and six embodiments describe thestructures and forming processes of the resin gears as the worm wheelsincorporated in the EPS apparatuses, but these structures and formingprocesses are not limited to worm wheels, and are applicable widely toresin gears.

As described above, in the resin gear wherein the thin resin is joinedto the entire outer peripheral surface of the teeth portion of thegear-shaped core metal, since this joining is carried out by way of thechemical bond in accordance with the chemically bonding compositemolding technique or the adhesive, the resin gear receives force applieddue to engagement of the gears as shearing stress of the joiningportion. Therefore, even though the layer of the resin is made thin, itsbreaking can be prevented to the utmost. Also, in the case of theinjection molding, when the high temperature resin is cooled andcontracted, residual tensile stress is generated, but a one brokenportion does not cause enlargement of the broken-out surface, and thebonding power is sustained to enhance its safety.

Further, as aluminum alloy or copper alloy is utilized for the coremetal, heat produced due to the engagement of the gears is easilydiffused, enabling its usage under comparatively high temperatureenvironment.

Particularly, when the core metal of the worm wheel is formed ofaluminum alloy and the worm wheel is incorporated in the EPS apparatus,the coefficient of thermal expansion of the aluminum alloy is the sameas that of the aluminum die-cast gear housing, change in backlash of thegears due to thermal expansion is reduced extremely, so that preferableoperation of the gears can be maintained invariably.

In comparison to a case wherein a gear is formed in a ring-like resin,the amount of used resin can be kept to a minimum. Therefore, thedimensional change due to water absorption can be reduced extremely,making it possible to secure smooth operation of the gears and to reducecost.

Furthermore, as it is structured that the core metal takes up the mostpart of the whole volume of the worm wheel, the strength of the gear canbe obtained by means of the core metal, so that the gear module can bereduced in size, contributing to achievement of miniaturization of theapparatus.

What is claimed is:
 1. An electrical power assisted steering apparatusprovided with a torque limiter having a ring member formed with aplurality of radially outwardly curved projections for applying elasticforce mounted between a worm wheel and an output shaft, wherein saidoutput shaft is made of iron material, a gear portion of said worm wheelis made of synthetic resin material, and a core metal portion of saidworm wheel is made of aluminum material which has a specific gravitysmaller than that of said iron material of the output shaft and whichhas a coefficient of linear thermal expansion larger than that of saidiron material of the output shaft, and wherein the gear portion and thecore metal portion are fixedly joined, whereby a change in spacingbetween said worm wheel and said output shaft as a result ofdifferential thermal expansion therebetween operates to change a limittorque of said torque limiter from a lower value under high temperatureto a higher value under low temperature.
 2. An electrical power assistedsteering apparatus according to claim 1, wherein said gear portion andsaid core portion are joined by way of chemical bond according to acomposite molding technique.
 3. An electrical power assisted steeringapparatus according to claim 1, wherein the outer peripheral surface ofthe core metal portion is formed with irregularities for strengtheningthe joining between said core metal portion and said resin gear portion.4. An electrical power assisted steering apparatus according to claim 3,wherein the outer peripheral surface of the core metal is toothed toprovide said irregularities.
 5. An electrical power assisted steeringapparatus comprising: a worm wheel formed with an axial through hole ata center thereof, an output shaft fitted in the axial through hole ofsaid worm wheel, and a torque limiter comprising a ring member formedwith a plurality of radially outwardly curved projections for applyingelastic force; wherein said output shaft is made of iron material, agear portion of said worm wheel is made of synthetic resin material, anda core metal portion of said worm wheel is made of aluminum materialwhich has a specific gravity smaller than that of said iron material ofthe output shaft, and which has a coefficient of linear thermalexpansion larger than that of said iron material of the output shaft,said gear portion and said core metal portion being fixedly joined,wherein said ring member of said torque limiter comprises asubstantially cylindrical thin metal member made of spring steel andformed with a plurality of radially outwardly curved projections, thesubstantially cylindrical thin metal member being press-fitted on theoutput shaft with said projections being in pressure contact with aninner peripheral surface defining said axial through hole of said wormwheel, and whereby a limit torque of said torque limiter will vary basedupon an operating temperature of said steering apparatus.
 6. Anelectrical power assisted steering apparatus according to claim 5,wherein said gear portion and said core portion are joined by way ofchemical bond according to a composite molding technique.
 7. Anelectrical power assisted steering apparatus according to claim 5,wherein the outer peripheral surface of the core metal portion is formedwith irregularities for strengthening the joining between said coremetal portion and said resin gear portion.
 8. An electrical powerassisted steering apparatus according to claim 7, wherein the outerperipheral surface of the core metal is toothed to provide saidirregularities.
 9. An electrical power assisted steering apparatuscomprising: a housing made of aluminum; an input shaft rotatablysupported in said housing and connected at one end to a steering wheel;an output shaft rotatably supported in said housing and connected at oneend to said input shaft and at the other end to a steering gear box, theoutput shaft being made of iron material; an electric motor for steeringassistance; a gear mechanism for transmitting rotation of said motor tosaid output shaft, the gear mechanism being received in said aluminumhousing and including a worm gear rotated by said motor, and a wormwheel with an axial throughhole formed in a center thereof, in whichsaid output shaft is fitted, the worm wheel being engaged with said wormgear; said worm wheel comprising a core metal portion made of aluminummaterial which has a specific gravity smaller than that of said ironmaterial of the output shaft and a coefficient of linear thermalexpansion larger than that of said iron material of the output shaft,and an outer peripheral annular gear portion made of synthetic resinmaterial and fixedly joined onto an outer peripheral surface of saidcore metal portion; and a torque limiter comprising a substantiallycylindrical thin metal member made of spring steel and formed with aplurality of radially outwardly curved projections, the substantiallycylindrical thin metal member being press-fitted on the output shaftwith said projections being in pressure contact with an inner peripheralsurface defining said axial throughhole of said worm wheel.
 10. Anelectrical power assisted steering apparatus according to claim 9,wherein said gear portion and said core portion are joined by way ofchemical bond according to a composite molding technique.
 11. Anelectrical power assisted steering apparatus according to claim 9,wherein the outer peripheral surface of the core metal portion is formedwith irregularities for strengthening the joining between said coremetal portion and said resin gear portion.
 12. An electrical powerassisted steering apparatus according to claim 11, wherein the outerperipheral surface of the core metal is toothed to provide saidirregularities.